Pythons can eat pretty large prey. Boas are the ones that you don't want thicker than their thickest part. BPs can go months without food and be fine. My girl goes from end of Oct to around Mar/Apr without food by her choice. She started just over a year old. When she was a couple month old baby, she ate anything that got near her face every 3 days. I was just feeding her F/T mice and then switched to F/T rats.

As for size, if you use a scale, you want something around 10% the snakes body weight. Easier is just feed it a rat or mouse that is as thick as its thickest part. Go to the pet shop so you can get an idea of the size you need. Just remember, most of the places will have size differences in their rodents. A small at one place might be a medium or large small at another place. If you only have 1 snake, my advice is not to order online. The shipping is expensive so unless you are ordering in big bulk or have a couple friends that you can all split up a large order, it will end up costing you more than just buying a single F/T at the pet shop every 1-2 weeks. I generally don't like to keep frozen rats more than 6 months but thats just me.